More recent automatic engraving methods involve the use of a numerically-controlled machine-tool with three working axes. For this purpose, the machine is equipped with a fine engraving tool chosen according to the material of the blank to be engraved. In such a machine, drive motors for the relative movements of the blank and tool are generally: stepping motors (with an open-loop control). The digital control signals of this machine are pulses of a given frequency, each pulse being designed to move the blank or the tool by one step along the horizontal axes X and Y and, when needed, along the vertical axis Z. The basic displacement step of the blank and/or the tool of precision machine-tools used in automatic engraving is generally equal to no more than one-hundredth of a millimeter.
In addition to computer-aided-design stations, the facilities generally used to produce such digital signals make use of a digitizing table associated with a computer. With such systems the operator can, by means of a drawing point, following the distinctive lines of the model to be reproduced, convert these lines into digital signals representative of the relative horizontal movements of the tool and of the blank to be engraved and then store these signals in the computer memory. Such means are altogether suitable for the automatic engraving of objects involving relatively simple graphic patterns. However, in the case of more complex models (for example, the features of a human face), the operator must have, in addition, certain artistic skills to be able to complete the model, first of all, and then follow the lines accurately with the drawing point. The engraving quality achieved will depend more on the skills of the operator than on anything else. Such a precondition obviously constitutes a significant impediment to the use of such an automatic engraving method for the production of objects with a faithful representation of a particular subject.
To cope with this particular difficulty, an automatic engraving method described in the U.S. Pat. No. 4,385,360 granted in 1983 to Yamada et al proposes the automatic entry of data relative to the image of the subject to be reproduced and, for this purpose, the use of a video camera associated with an analog-digital converter. This method is however limited in that it does not enable the manufacture of objects of a quality comparable to that obtained using the previously described method. In fact, in the method of the cited patent, engraving is carried out by means of a large tapered-head milling cutter and is designed to produce as hollows, in a generally transparent plastic blank, a limited number of points with a diameter varying according to the gray levels of the corresponding points of the image of the subject presented to the camera. Although relatively faithful, the result is of poor esthetic quality. On the other hand, with the automatic engraving method using a digitizing table, with a skilled operator, good manual data entry is possible thus allowing the engraving of continuous lines faithfully reproducing the distinctive lines of contours and contrasts in the image of the subject to be represented. This is moreover done on any flat, opaque or transparent substrate.